Mountains of the Moon tells the story of a woman in her 30s, recently released from prison, who finds herself a job filling doughnuts. But IJ Kay's extraordinary debut also tells the story of Lulu, aged seven, escaping the abuse of her mother's home for "Africa", a nearby wasteland, prowling with hyenas and jackals and the dangerous Sandwich Man. Then there's Catherine, aged 11, in a young offenders institution; teenage Mitten, gloriously, hopelessly in love; and Kim, a 21-year-old croupier, witness to a shocking crime, betrayed by those closest to her.

Each of these characters tells her own story in fragments the reader must piece together to gain a sense of the single, nameless woman who encompasses them all. This is a novel about one woman's struggle for existence, both physical and psychological, and in spite of the desperate subject matter it is above all a triumphant, uplifting expression of an individual's capacity to transcend the brutality and ugliness of everyday life and create something unique and magnificent.

Mountains of the Moon is not in any sense an easy read. Kay makes great demands on the reader, and apart from a handy cast list at the beginning, offers few concessions to those struggling to weave the various strands into a vaguely comprehensible whole. It is a bold decision, especially for a debut novelist, and one that I fear may deter some readers, but those who persist will be well rewarded. By the end of a second reading – and it took me two readings, I'll admit – there's a sense of having experienced something genuinely unforgettable.

The voice of the narrator is integral both to the themes of the book and to its impact on the reader. While adapting to each identity, it remains unmistakably her own and offers the reader a means of linking the fragments together. It's a wonderful voice: funny and fragile, innocent, knowing, tender and tough. I have never encountered another like it. As the narrator explains to the casually callous Gwen, "No one speaks like me in real life. If I didn't speak the way I does I wouldn't even exist." Quite so. For much of the story Lulu/Catherine/Mitten's voice is the only thing she has.

The narrator's struggle to believe she exists, to forge and sustain her own existence, makes for a moving narrative. Her mother is the first to try to stamp it out. She is incapable of recognising her daughter's existence, except in relation to her own. "'She speaks badly on purpose,' Mum says, 'just to show me up.'" On another occasion, "She feels down the back of my legs and I int even a horse. 'Dancer's legs like mine,' she says."

Lulu learns to split in two, to stand outside herself. In one powerful description of a car journey, she imagines herself outside the car: "Here I is, here I is, running, running on the verge. Now and then I look to see, case I'm still sitting in the car, looking out of the winder. I smiles. Sometimes I run close enough for one hand fingers on the glass.

I am real."

It's a survival strategy and an effective one, but it leaves a legacy. Love necessitates self-revelation, but when the glorious Velvit Gentleman asks the teenage Mitten who she is, she is unable to give him an answer.

"'So who are you?'

'Who do you want me to be?'"

The narrator's journey takes her from the "Africa" of her childhood, with its lamp-post giraffes and bulldozer elephants, to the "real life" Mountains of the Moon in Uganda, where imagination and reality collide in spectacular fashion. This is not a Cinderella story. Kay is too intelligent a writer to give us the happy ever after. But for all its devastating subject matter, Mountains of the Moon is nonetheless full of hope. It is notable how much kindness there is in it. Right at the start a station-café owner gives the narrator food and drink on learning that she's just come out of prison. And this is echoed throughout the novel; acts of kindness from random strangers, the tenderness between Lulu and her siblings, the real love of her grandparents, the fashion student who gives Kim her precious coat. There is nothing simplistic about this bold, unsettling, uplifting novel. Read it. Then read it again.